A MEMS micro-mirror device is a device that contains an optical MEMS (Micro-Electrical-Mechanical-System). The optical MEMS may comprise a cylindrical, rectangular or square micro-mirror that is adapted to move and to deflect light over time. The micro-mirror is usually connected by torsion arms to a fixed part and can tilt and oscillate along one or two axis. Different actuation principles can be used, including electrostatic, thermal, electro-magnetic or piezo-electric. MEMS micro-mirror devices are known in which the area of these micro-mirrors are around a few mm2. In this case, the dimensions of the MEMS device, comprising the packaging, is around ten mm2. This device is usually made of silicon, and can be encapsulated in a package that can include the driving actuation electronics. Various optical components, such as for example lenses, beam combiner, quarter-wave plates, beam splitter and laser chips, are assembled with the packaged MEMS to build a complete system such as, for example, a projection system.
A typical application of the MEMS micro-mirror devices is for projection systems. In a projection system, a 2-D image or a video can be displayed on a display surface; each pixel of the 2-D image or a video is generated by combining modulated red, green and blue laser light sources, by means of, for example, a beam combiner. The combined light from the modulated red, green and blue laser is emitted from the beam combiner as a beam of light. The beam of light emitted from the beam combiner comprises pulses, and each pulse will correspond to a pixel of the 2-D image or a video. A MEMS micro-mirror device directs the beam of light to a display surface and oscillates to scan the beam of light in a zig-zag or lissajou pattern across the display surface so that the 2-D image, or a video, is displayed on the display surface, pixel-by-pixel. The micro-mirror within the MEMS micro-mirror device will continuously scan light from left to right and from top to bottom so that each pixel of the 2-D image or a video is continuously refreshed. The speed of oscillation micro-mirror is such that a complete 2-D image or a video is visible on the display surface.
Typically, the micro-mirror of a MEMS micro-mirror device is able to oscillate along one axis. Therefore, in order to display a 2-D image on a display surface a projection system will require two MEMS micro-mirror devices; a first MEMS micro-mirror device is required to scan light along the horizontal and a second MEMS micro-mirror device is required to scan light along the vertical. The first and the second MEMS micro-mirror devices must be precisely positioned such that the oscillatory axes of their respective micro-mirrors are orthogonal.
During operation, the micro-mirror of the first MEMS micro-mirror device receives light from the beam combiner and deflects the light to the micro-mirror of the second MEMS micro-mirror device. The micro-mirror of the second MEMS micro-mirror device will in turn deflect the light to the display surface where it will appear as a pixel. The micro-mirror of the first MEMS micro-mirror device will oscillate to scan the light along the horizontal thereby displaying the first row of pixels on the display surface, pixel by pixel. When the first row of pixels have been projected onto the display surface, the micro-mirror of the second MEMS micro-mirror device will move about its oscillatory axis so that light received from the micro-mirror of the first MEMS micro-mirror device is directed towards the next row where pixels are to be displayed. The micro-mirror of the first MEMS micro-mirror device will then oscillate to scan the light from along the horizontal to display the next row of pixels. The process is continuous so that a complete image is visible on the display surface. Typically, the speed of oscillation of the micro-mirror in the second MEMS micro-mirror device will be much slower than the speed of oscillation of the micro-mirror in the first MEMS micro-mirror device. Accordingly, the micro mirror in the second MEMS micro-mirror device (i.e., the micro-mirror which is responsible for scanning the light along the vertical) is often referred to as the ‘slow mirror’ and the micro-mirror in the first MEMS micro-mirror device (i.e., the micro-mirror which is responsible for scanning the light along the horizontal) is often referred to as the ‘fast mirror’.
It is also known to provide the fast and slow micro mirror within the same MEMS micro-mirror device. Advantageously, with such MEMS micro-mirror devices the micro-mirrors are pre-arranged during the manufacturing stage within the MEMS micro-mirror device such that their oscillatory axes are orthogonal. A further advantage is that a projection system will require only one such MEMS micro-mirror device to display a 2-D image on a display surface.
Other MEMS micro-mirror devices comprise a single 2-D micro-mirror which can oscillate along two orthogonal oscillation axes. During operation, the single 2-D micro-mirror receives modulated light from the beam combiner and deflects the light to a display surface where it will appear as a pixel. The single 2-D micro-mirror will oscillate along a first oscillation axis to scan the light along the horizontal thereby displaying the first row of pixels on the display surface. When the first row of pixels are have been projected onto the display surface, the single 2-D micro-mirror oscillates about a second oscillation axis (which is orthogonal to the first oscillation axis) so that light received from the beam combiner is directed towards the next row where pixels are to be displayed. The single 2-D micro-mirror will oscillate along the first oscillation axis to scan the light from the beam combiner along the horizontal thereby displaying the next row of pixels on the display surface. The process is continuous so that a complete image is visible on the display surface. It is also possible that the 2-D micro-mirror oscillates about both the first and second oscillation axis simultaneously. The advantage of using a single 2-D micro-mirror which can oscillate along two orthogonal oscillation axes, is that only a single micro-mirror is required to display a 2-D image on a display surface. Typically, the speed of oscillation of the single 2-D micro-mirror about the first oscillation axis is much greater than the speed of oscillation of the single 2-D micro-mirror about the second oscillation axis; accordingly, the first oscillation axis (i.e., the axis about which the single 2-D micro-mirror oscillates to scan light along the horizontal) is known as the “fast axis” and the second oscillation axis (i.e., the axis about which the single 2-D micro-mirror oscillates to scan light along the vertical) is known as the “slow axis”.
The speed of oscillation of the micro-mirrors about their respective oscillation axes greatly impacts on the quality of the projected image visible on the display surface. For example, if the fast mirror is oscillated about its oscillation axis too quickly, then the spacing between consecutive pixels on the display surface will be too large and the projected image will appear dull on the display surface. Conversely, if the fast mirror is oscillated about its oscillation axis too slowly, then overlapping of consecutive pixels may occur on the display surface and the quality of the projected image visible on the display surface will be compromised.
Usually, the “fast mirror” oscillates at its mechanical resonant frequency of oscillation, or in the case of the single 2-D micro-mirror it usually oscillates at its mechanical resonant frequency of oscillation about the “fast axis”. Therefore, the speed of oscillation of the “fast mirror”, or single 2-D micro-mirror about the “fast axis”, cannot be increased any further without compromising with other characteristics (power consumption, scanning angle). In contrast the speed of oscillation of the “slow mirror”, and the speed of oscillation of the single 2-D micro-mirror about the “slow axis”, can be increased and manipulated.
The speed of oscillation of the “slow mirror” is preferably such that as the fast mirror scans along a row the slow mirror oscillates very slowly such that the light is scanned in a zig-zag pattern across the display screen.
The fast mirror and slow mirror should continue to oscillate until each pixel of the 2-D image or video has been projected to the display surface. The process of scanning light from the projector over the display surface is continuously repeated and is carried out at a speed which will ensure that a complete image is visible on the display surface. Thus, once the light from the projector has been scanned over the display surface to display each of the pixels of the 2-D image or video, the light from the projector must again be projected towards the top of the image so that scanning process may begin once again so that the projected image can be “refreshed”. To direct the light to the top of the image once more the slow mirror must oscillate so that it returns to its original position. Preferably, the slow mirror should oscillate instantaneously back to its original position. Thus, ideally the amplitude of oscillation of the slow mirror should have a saw-tooth profile as depicted in FIG. 1.
Between time points A and B the slow mirror oscillates slowly to scan the light vertically along the display surface. The combination of vertical scanning provided by the slow mirror and horizontal scanning provided by the fast mirror, means that light from the projector is scanned in a zig-zag pattern over the display surface. Alternatively, the slow mirror may be oscillated stepwise, as shown in FIG. 2; in this case each row of pixels will be projected along a horizontal line, row by row; the fast mirror oscillates to scan light along the horizontal to display a row of pixels; once a row of pixels have been projected the slow mirror will oscillate so that projected light is directed to the next row where pixels are to be displayed. The number of steps which the slow mirror undertakes will correspond to the number of rows which form the projected image; the number of steps is usually approximately between 240 and 1080.
Regardless of whether the slow mirror undergoes a constant slow oscillating movement or a stepwise oscillation, at point B each pixel of the 2-D image or video has been projected onto the display surface. Thus at B the slow mirror instantaneously oscillates back to its original position so that the scanning process can once again begin from the first pixel of the 2-D image so that the projected image can be “refreshed”.
However, in reality, due to the inertia and mass of the slow mirror, and friction, the actuator which oscillates the slow mirror cannot oscillate the slow mirror instantaneously back to its original position. Accordingly, the profile of the amplitude of oscillation of the slow mirror will have a rise/fall time ‘h’ as depicted in FIG. 3. Normally the amplitude of oscillation of the slow mirror will have a 10% rise time, or fall time, with respectively a 90% fall time, or rise time.
As depicted in FIG. 4, if the rise time or fall time of actuating signal used to actuate oscillation of the slow mirror is too short or too long, then stray oscillations 2 may be imparted on the slow mirror. The stray oscillations can be due to rebound of the slow mirror for example. Furthermore, as the slow mirror is subject to a low air damping, a step response of the slow mirror will be subject to stray oscillations. Moreover, these stray oscillations will increase with decreasing air damping applied on a surface of the slow mirror.
As illustrated in FIG. 5, the stray oscillations 2 compromise the projected image 3 visible on the display surface 5 as parts of the projected image 3 will appear brighter than other parts. For example, FIG. 4 shows that part 3a of the projected image 3 is brighter than part 3b of the projected image 3.
The US patent application US2008204839 describes a system which uses a vertical scanning wave (W) (for each period) which includes a saw tooth wave (Wa) part followed by a corrective wave part (Wb). The scanning wave is applied to a reflective surface. Disadvantageously, the scanning wave requires a corrective part to suppress stray oscillations of the reflecting plate caused when the saw tooth wave instantly returning from the maximum level to the minimum level.
It is an aim of the present invention to obviate or mitigate one or more of the aforementioned disadvantages.